The functional block diagram of FIG. 1 shows a basic form of an RO (reverse osmosis) filtering system of known art without energy recovery, e.g. a small system for an individual home or small business. A pressurized RO enclosure 10 receives a water supply at its input node “a” at elevated pressure, either from municipal water mains or, as shown pressurized by a pressure pump 12 driven by a motor 14, typically electric. Pump 12 draws water from a relatively low pressure source such as a well, river or lake optionally via preconditioning and filtration apparatus, and develops high pressure at node “b”, the intake port of the main chamber 10A of RO container 10, where the pressurized liquid is forced against an RO membrane 10B, typically polyamide thin-film composite that will not pass sodium or chloride ions. In RO seawater desalination, about 40% of the intake liquid traverses membrane 10B to compartment 10C as desalinated water, available to be drawn off as required at outlet “c”. The remaining 60% of the input liquid including extracted residue leaves chamber 10A as a secondary flow of more concentrated unwanted components, such as the salt in brine, from the RO outlet port “d” where it passes through a back-pressure regulating valve 17 to discharge port “e” where the secondary flow is discharged to a drainage system as wastewater, typically returned directly or indirectly to the sea.
It has long been recognized that there is a substantial amount of energy Ed available in the secondary liquid flow at RO brine exit port “d”, where, compared to 100% pressure Pb and flow rate Rb of the primary flow at intake port “b”, the pressure Pd is typically 99% and the flow rate Rd is 60%.
The energy at the RO brine exit port “d” can be calculated from the product of pressure and flow rate (Ed=Pd*Rd): for the foregoing conditions, Ed is found to be 59.4% of Eb. Since the ultimate discharge from node “e” is typically at relatively low pressure, most of the waste energy is dissipated as heat at valve 17 and its environment. Efficient recovery of this energy can provide substantial savings in operating cost.
The energy available at the outlet valve 17 can be estimated from the reduction in pressure at the rated discharge flow; if this energy could be totally exchanged for a reduction in the electrical energy consumed by the motor driving the primary pump 12, the net energy recovery of 59.4% would reduce the operating energy cost to 40.6% of the operating energy cost of the basic non-recovery system of FIG. 1. However, practical systems can only approach this limit due to unavoidable machine losses such as friction of bearings, and pistons, and leakage of seals, pistons and valves, turbulence, etc. Thus the design of more efficient systems of this kind remains a challenge with potential that has not yet been fulfilled by known art.
FIG. 2 illustrates a system as in FIG. 1 modified to include energy recovery by the addition of an energy exchanger 18, connected into the RO brine discharge flow path between nodes “d” and “e” in place of valve 17. Energy recaptured from pressure drop in this flow is fed back to the primary liquid flow side between nodes “a” and “b” as indicated by the arrow.
Many different approaches have been suggested and tried for implementing this energy feedback. The flow/pressure drop energy recaptured in a liquid motor such a turbine from which torque can be applied to shaft 16 of the primary RO pressurizing pump 12. Alternatively or additionally, the recovered energy torque can drive an auxiliary pump or equivalent introduced in series and/or parallel with the existing primary pump 12 to reduce its pressure/flow rate loading, and thus reduce the electric power consumption of drive motor 14. The efficiency of this energy exchange system is critically important since it directly affects the actual amount of operating cost savings realized. Electric motor efficiency is about 90-95% and pump efficiency ranges from 50 to 90%, typically 80%, so these machines are generally selected for high efficiency.
Energy and pressure exchange systems have been the subject of much design research, development and refinement to reduce capital costs and operating costs; with increasing concern about world wide consumer water availability, there are increasing efforts to develop machines that recapture energy from RO brine discharge even more efficiently to accomplish more cost-effective desalination.